Along with the increasingly smaller sizes and greater number of functions of electronic equipment, greater density of the circuit boards used in the electronic equipment is also being sought. The general method for increasing the density of circuit boards is to make the interconnections finer, that is, reduce the line widths. As insulation materials used for this purpose, curable compositions superior in low dielectric property, low water absorption, and heat resistance are being sought. For example, International Publication WO98/56011 discloses the following curable composition. This curable composition is obtained by filtering a graft polymer containing a cyclic structure by a filter of a pore size of 0.22 μm, then dissolving or dispersing it in an organic solvent together with a curing agent. The graft polymer containing a cyclic structure is produced by graft modifying a norbornene-based polymer or other polymer containing a cyclic structure in xylene or another hydrocarbon-based solvent in the presence of a large amount of peroxide by maleic anhydride or another unsaturated compound by 2.5 mol %.
On the other hand, in applications of electronic equipment, there has been a sharp increase in fields where a high heat resistance is sought in the insulating materials as well and fields where surface hardness for multilayer structures are sought. To deal with this, in the curable composition described in the above international publication, a graft polymer containing a cyclic structure having a high graft ratio has to be used. The graft polymer containing a cyclic structure of the high graft ratio described in that international publication improves the heat resistance and the surface hardness, but has a large gel content, so the solution of the polymer dissolved in a solvent cannot be filtered. Therefore, the coating film has the defects of insufficient smoothness and flatness.